Mercury
is the planet closest to the blast furnace of the Sun.
It's a small planet, only slightly larger than our Moon.
Its "day" is nearly as long as its "year". At high noon, the temperature
is high enough to melt lead and zinc at its Equator. But the North Pole
seems to be made of ice. Mercury has a wrinkled surface because it seems
to have shrunk, like an apple drying in the Sun.

Mercury has always been hard
to look at, because it is so close to the Sun. You can see Mercury for
less than two hours before sunrise, or less than two hours after sunset.
So the ancient astronomers actually believed that Mercury was two planets.
Mercury was the planet seen in the early evenings, while Apollo was the
planet seen in the early morning. The Romans called this planet after
Mercury, the wing-footed messenger of the gods, because it moved so fast
through the sky.

The planet Mercury is just
a ball bearing of iron, with a thin layer of rock. About 80% of the diameter
of the whole planet is taken up by its core of metallic iron. For some
unknown reason, Mercury is linked to the Earth.
Every time Mercury reaches its closest distance to us (every 117 days)
it always shows the same face to us.

On Earth, the Sun appears to
sweep across high noon every 24 hours. We call this period of 24 hours
a "day". It takes our planet Earth about 365 days to go right around the
Sun back to where it first started from. On Mercury, the days last about
59 Earth days. It takes Mercury about 88 Earth days to have a complete
year. So there are three Mercury days every two Mercury years. The Mercury
midday is very strange. If you were standing on the equator of Mercury
you would see the Sun slow down as it moved towards high noon, then stop,
then start up again in the opposite direction.

Only one spacecraft has ever
visited Mercury. The Mariner 10 made three fly-pasts of Mercury in 1974/5.
It took photographs of 45% of Mercury's surface. The main things the Mariner
found on Mercury were craters and cliffs.

There are small craters less
than 10 km across. They have bowl-shaped bottoms and they were caused
by impacts with meteorites weighing from 10,000 to 100,000 tonnes. Craters
between 10 and 20 km across have a flat bottom. Once the craters get bigger
than 20 kilometres, they all have a flat bottom and a peak in the centre
of the crater. Craters between 20 and 150 km across are caused by meteorites
weighing between 1 and 100 billion tonnes. As the crater gets closer to
150 kilometres across, the central peak becomes taller and wider. When
the craters are 150 to 200 km across, the peak gets so large that it turns
into a central ring.

The Caloris Basin is 1,300
km across. It was caused by an enormous rock slamming into Mercury. The
impact was so colossal that shock waves ran through the planet and focused
to an area about 1,000 kilometres across, exactly on the other side of
the planet. The ground was shaken like a giant pair of hands would shake
dust out of a rug, and that's how the huge Caloris Basin was made.

The other major feature on
Mercury are the cliffs. These can be a kilometre or two high, and run
for hundreds of kilometres.

Mercury was born in giant collisions
about four and a half billion years ago, as huge rocks collided together.
Then there was a time of several hundred million years of intense bombardment
by massive meteorites, and there was much volcanic activity. As the planet
cooled down, the rocky crust shrank. Overall, Mercury shrank in diameter
by about 3 kilometres. The cliffs were caused by the crumpling of the
crust as Mercury cooled. The meteorite bombardments gradually lessened
until the Caloris Basin was formed in a colossal impact about 3.8 billion
years ago. Caloris is named from the word "calor" meaning heat. So much
heat was dumped into the planet's crust that volcanic activity began again
for a short while. But then it stopped, and there has been no major geological
activity for the last 3.8 billion years.

Mercury is a very hot planet,
because it is so close to the searing heat of the Sun. It gets 10 times
as much solar radiation as our planet. Around midday of the long day,
the temperature can reach 430OC, hot enough to melt lead and zinc. There
is hardly any atmosphere to carry the heat around to the night side of
the planet, so the temperature there can plummet to -170OC.

Some scientists think that
Mercury has ice at its north pole. Not much sunlight reaches the north
pole, and the radar photographs show something that looks like ice covering
an area of about 400 square kilometres. The atmosphere of Mercury is so
thin that its hardly there. It's about 1 trillionth (1 millionth of a
million) as thick as our atmosphere on Earth. It's almost a perfect vacuum,
though we have found traces of helium and hydrogen, as well as sodium,
potassium and oxygen.

Our first series of fly-pasts
of Mercury answered many questions, but it raised many more. It is time
for another visit by one of our robot explorers.